1. Field of the Invention
The invention relates to multilayer polyolefin film structures and methods of making the same and, more particularly, the invention relates to a biaxially-oriented polypropylene film incorporating at least one polyester layer.
2. Description of Related Art
Biaxially-oriented polypropylene (BOPP) films are widely used in packaging because they have good stiffness, strength, optical properties (low haze and high gloss), and moisture barrier properties. Users of packaging films, particularly users of biaxially oriented polypropylene films, are continually seeking structures with improved printability, metallizing properties, and gas barrier. Because of their olefinic nature, typical BOPP constructions have low surface energy and require treatment (corona, flame, etc.) in order to be printed or metallized. Polyester is known to have high surface energy and possesses excellent printing and metallizing attributes. Additionally, polyester, both in clear and metallized structures offers improved gas barrier performance to BOPP films. This is especially true in the case of metallized polyester films which are in order of magnitude or more lower in oxygen transmission rate.
Although there has been a long felt need for BOPP films incorporating polyester layers, problems in obtaining acceptable optical and processability characteristics, adequate interply adhesion, and other properties have been encountered in previous attempts to produce polyester-containing BOPP structures. For example, U.S. Pat. No. 5,324,467 discloses a process for the preparation of an oriented multilayer laminate film having at least three layers, including polypropylene, a tie layer, and copolyester. The films are formed by combining the layers in the molten state, either in coextrusion, or in separate extrusions brought together outside the die, then subsequently cooling the film, orienting it uniaxially or biaxially, and heat setting to lock in the properties. A major problem in producing a structure according to this method on commercial scale equipment is the strong tendency of polyester to adhere to the heated metal rolls of the machine direction orientation section. This makes it difficult to achieve good optical properties free of visual defects and may also decrease other properties such as the seal initiation temperature.
U.S. Pat. No. 4,874,656 describes a multilayer laminate having a high mechanical resistance and an impermeability to gases and vapors. In the disclosed structures, a polyester layer is joined to a polypropylene layer after the polypropylene is biaxially-oriented, the polyester layer is quite thick (i.e., 12 to 24 microns), and the structure includes a layer of metallic foil and a layer of polyethylene. While joining a polyester layer to a BOPP layer after biaxial orientation is possible, this method is impractical for incorporating thin layers of polyester.
U.S. Pat. No. 4,924,525 also describes a structure wherein a polyester laminate is adhered to a BOPP film after the polypropylene is biaxially oriented, precluding the use of thin polyester layers in the final structure.
It is an object of the invention to overcome one or more of the problems described above.
Accordingly, the invention provides a biaxially-oriented polypropylene film incorporating a polyester layer, and a method of making the same.
The inventive multilayer film is prepared by the steps of forming a polypropylene core, orienting the core in a first direction, providing on at least one side thereof a multilayer outer film (cap layer) comprising at least one polyester layer and at least one tie layer interposed between the polyester layer and the core, and orienting the resulting multilayer film in a second direction transverse to the first direction.
Advantageously, the polyester layer contains sufficient silicone fluid to provide substantially uniform stretching characteristics.
In one embodiment of the invention, an outer surface of the multilayer film opposite the polyester layer is metallized. In another embodiment, a tie layer adhesive is an ethylene/carboxylic acid or anhydride copolymer or an ethylene/ester/carboxylic acid or anhydride terpolymer.
The invention further provides a white or colored biaxially oriented polypropylene film incorporating a polyester layer that is prepared by incorporating inorganic minerals in the polypropylene core layer.
If desired, an additive such as a pigment, dye, etc. may be incorporated into the multilayer film.
In yet another embodiment, the core layer comprises a polypropylene film coextruded between first and second polyolefin layers.
Further objects and advantages of the invention will be apparent to those skilled in the art from a review of the following detailed description taken in conjunction with the appended claims.
The invention addresses various concerns of the prior art by providing a structure that positions the polyester at the outer layer of the film, thereby taking advantage of the improved printability and metallizing attributes of polyesters, also eliminates the difficulty of contacting polyester over the heated rolls of the machine direction orienter and, further, provides thin polyester layers that allow a structure of economic value to be produced. An additional benefit of the invention is that a broad range of polyester products, including amorphous homopolymer grades, may be included in the inventive film structures. This allows the designer a wide choice in making films with improved optical properties, printability, and metallizing attributes as well as stiffness and heat resistance.
The films of the present invention provide excellent barrier to flavors and aromas. Moreover, since the polyester layer of the inventive film is formed from an extruded high molecular weight polymer, there is no problem with loss of flavor or aroma barrier due to cracking or abrasion. Furthermore, BOPP films with an external polyester layer can be readily printed or metallized, or adhesively coated on the polyester surface. Also, the film surface opposite the polyester side can be metallized, leaving the polyester layer available for other modifications.
Generally, the polyester-containing BOPP film of the invention includes a core and a multilayer outer film (cap layer) or film adhered to at least one surface of the core. The multilayer BOPP film of the invention is prepared using interdraw coating or lamination techniques.
The BOPP core comprises a layer of polypropylene and, in one embodiment, further comprises a tie layer, as described below.
The cap layer is applied on one or both surfaces of the monoaxially oriented core layer. The cap layer comprises a layer of a polyester resin and at least one tie layer. The tie layer is disposed between the polyester layer and the core.
Additionally, the polyester layer contains a sufficient amount of silicone fluid as a processing aid to provide substantially uniform stretching characteristics to the polyester layer.
In a preferred embodiment, the cap layer contains a second tie layer comprising a polypropylene copolymer or terpolymer or linear ethylene polymer interposed between the adhesive tie layer and the core. In a variation on this embodiment, the polypropylene copolymer or terpolymer tie layer forms part of the core, and is positioned adjacent the polyolefin adhesive tie layer in the final structure. This tie layer may be oriented with the polypropylene layer of the core.
The multilayer BOPP film of the invention is prepared by the steps of forming and orienting the core in a first direction, providing the cap layer to at least one side of the monoaxially oriented core to form a multilayer film, and then orienting the resulting multilayer film in a second direction transverse (and preferably perpendicular) to the first direction to provide a biaxially-oriented multilayer film.
The biaxially-oriented multilayer film may then be subjected to a heat setting treatment to allow the film to crystallize. In a preferred embodiment, an outer surface of the polyester layer and/or an outer surface of the multilayer film opposite the polyester layer is metallized.
The invention is described in more detail below.
As stated above, the core may be a polypropylene monolayer or may comprise a multilayer structure including a core layer of a polypropylene with a tie layer on one or both sides of the core.
The term xe2x80x9cpolypropylenexe2x80x9d as used herein with reference to the core generically denotes a semi-crystalline polymer with a majority of polymerized propylene, and specifically includes isotactic homopolymers of propylene, copolymers of propylene with up to 25 weight percent ethylene or butene, terpolymers of propylene with ethylene and butene, and mixtures thereof.
Preferred polypropylenes are those selected from propylene homopolymers and copolymers of propylene with less than three weight percent comonomer such as ethylene or butene. Melt flow rates of 1 to 15 dg/min, and preferably from 1.5 to 6 dg/min, as measured according to ASTM D1238-90b, Condition 230/2.16 (formerly Condition F) are suitable for sheet or blown film.
The thickness of the core layer is limited only as dictated by oriented polypropylene tenter process limitations, and typically will range from about 12 microns to about 50 microns.
The core may optionally include a tie layer comprising a polypropylene copolymer or terpolymer or a linear ethylene polymer coextruded with the polypropylene core layer. While the polypropylene core may be a homopolymer, the coextruded tie layer comprises a copolymer of propylene with up to 25 weight percent of ethylene or butene, mixtures thereof, or a linear ethylene polymer such as linear low density polyethylene (LLDPE). The thickness of the total core structure is limited only by the tenter process limitations as described above and thus is typically about 12 microns to about 50 microns thick. The thickness of an individual coextruded tie layer is typically about 0.5 microns to about 2 microns thick.
One important class of BOPP films are white, pigmented films used in packaging applications. For example, confectionery goods are frequently packaged with white BOPP films because the films provide a light barrier to prevent premature spoilage initiated by UV light and the white films present a clean, appealing surface.
For white film versions made according to the invention, the clear film core structure is modified by the addition of incompatible inorganic minerals. An especially important mineral is titanium dioxide, TiO2, the most commonly used white pigment. Typical TiO2 concentration range in the core is about one weight percent to about 20 weight percent. Thicker films require less Tio2 to attain the same whiteness. For the inventive films, the preferred concentration range is about four weight percent to about 15 weight percent.
Other minerals that may be used are aluminum oxide, zinc oxide (ZnO), calcium sulfate, barium sulfate, calcium carbonate (e.g, chalk), magnesium carbonate, sodium silicate, aluminum silicate, silicon dioxide (SiO2, i.e., silica), mica, clay, talc, and the like in a range of about two weight percent to about 25 weight percent in the core. The action of these minerals is to cause formation of cavities or voids in the film. These cavities contribute to making the film more opaque due to multiple light scattering. The concentration of the minerals and their particle sizes help determine the void structure and several film properties.
Other additives can be used, such as antioxidants, lubricants, surfactants, antistats, slip agent, antiblock agents, nucleating agents, coupling agents, and coated minerals. Similarly, addition of pigments and dyes (inorganic and organic) to the core or encapsulating coextruded layers of the white versions can yield colors other than white.
Addition of the minerals may be accomplished by using a separate feed stream of mineral into the extruder that produces the core polypropylene melt layer, or by initially blending a dry mix of the mineral and polypropylene and then extruding the mixture, or by masterbatch concentrate. Masterbatch concentrates of the minerals in polypropylene are first melt compounded. These concentrates are then separately added to the core extruder feed with the polypropylene.
In one embodiment, the white film core structure comprises three coextruded layers. The center layer (typically 10 microns to 50 microns in thickness), is encapsulated by two outer coextruded polyolefin layers. These typically 0.5 microns to 5 microns thick encapsulating layers provide continuous non-porous layers. Minerals may be contained in any of the core layers. In one form, the center layer contains the whitening minerals, and the encapsulating layers may contain TiO2 to enhance the whitening power. In another version, these encapsulating layers may contain TiO2 and cavitating minerals such as CaCO3, for example, whereas the middle layer is free of TiO2 or other minerals.
If desired, a white film may have a metallized outer surface, on an outer surface of the polyester layer and/or an outer surface of the multilayer film opposite the polyester layer.
The cap layer comprises a two-layer or three-layer film, including an outer, polyester layer and a first tie layer comprising a polar or grafted olefin polymer adhesive. Preferably, the cap layer further includes a second tie layer comprising a copolymer of propylene with up to 25 weight percent of ethylene or butene, a terpolymer of propylene, ethylene, adhesive, mixtures thereof, or a linear ethylene polymer, such as LLDPE.
The polyester layer comprises a crystalline copolyester, a crystallizable amorphous polyester homopolymer, or a crystallizable amorphous copolyester. (The terms xe2x80x9ccrystallinexe2x80x9d and xe2x80x9camorphousxe2x80x9d describe the solid state structure of the polyester as supplied by the vendor and prior to orientation.)
By the term xe2x80x9ccopolyesterxe2x80x9d it is meant that the polyester is the reaction product of at least one polyol and one carboxylic acid, with there being a total of at least three monomers selected from the polyols and acids. xe2x80x9cHomopolymerxe2x80x9d polyesters are understood to include a single polyol and a single acid moiety.
The polyester layer contains, as a processing aid, a sufficient concentration of a silicone fluid (i.e., a dimethyl polysiloxane or equivalent), preferably of a high molecular weight (e.g., having a viscosity in the range of 300,000 cps to about 2,000,000 cps, highly preferably about 1,000,000 cps as measured by Brookfield viscometer) in an amount sufficient to provide uniform polyester stretching characteristics. Typically, a polyester layer intended for subsequent metallization will contain about 1,000 ppm to about 3,000 ppm silicone fluid, preferably about 1,000 ppm to about 2,000 ppm (based on the weight of the polyester) in the polyester layer. Polyesters not intended for metallization may contain higher concentrations (e.g., up to about one weight percent) of silicone fluid, if desired. (Silicone fluid concentrations greater than about one weight percent leads to intermittent, non-steady state extrusion.)
Baysilone silicone fluid M 1,000,000 is preferred silicone fluid.
The silicone fluid may be added to the polyester by intensive mixing with pellets of polyester resin in order to coat the pellets, followed by drying of the coated pellets, and extrusion. A Henschel mixer is suitable for preparing the silicone fluid-coated pellets.
The adhesive tie layer adhered to the polyester layer may comprise a copolymer of ethylene with an ester such as an ethylene/vinyl acetate copolymer, or an ethylene/methyl acrylate copolymer, an ethylene/n-butyl acrylate copolymer, or an ethylene/ethyl acrylate copolymer, for example. Ionomers (partially hydrolyzed ester derivatives) are also suitable comonomers. Alternatively, the first tie layer may comprise a grafted polyolefin adhesive, such as a polyethylene or polypropylene backbone grafted with at least one ethylenically unsaturated carboxylic acid, carboxylic acid anhydride, or other derivative, as known in the art.
Either of the first and second tie layers may be a copolymer of ethylene and a carboxylic acid or carboxylic acid anhydride, or a terpolymer of ethylene, an ester, and a carboxylic acid or carboxylic acid or anhydride. Suitable carboxylic acids and carboxylic acid anhydrides include, but are not limited to acrylic acid, methacrylic acid, and maleic acid or maleic anhydride (the latter generally used as grafting monomers).
Suitable terpolymers may have the ethylene, ester, and acid or anhydride incorporated 28 into a main chain of the polymer, or may comprise an ethylene/ester copolymer grafted with the acid or anhydride.
Preferred adhesives include maleic anhydride modified ethylene-vinyl acetate, such as Bynel(copyright) E418 adhesive resin available from DuPont, and Escor(copyright) ATX 325 acid terpolymer available from Exxon Chemical, which is an ethylene-based resin having both ester and acrylic acid functionality.
In a preferred embodiment, the cap layer further comprises a second tie layer comprising a propylene copolymer or terpolymer or a linear ethylene polymer such as LLDPE, as described above. In this embodiment, the ethylene/ester copolymer or grafted polyolefin adhesive is interposed between the second tie layer and the polyester layer.
The respective thicknesses of the polyester layer, first tie layer, and second tie layer may vary within wide ranges, and are substantially independent of each other. Typical approximate thicknesses for the polyester, first tie layer, and second tie layer in the final film are as follows:
Preferably, the total thickness of the cap layer is in the range of about 1.5 microns to about 6.5 microns in the final film.
The cap layer may be formed by any suitable process, including blown or cast film coextrusion, as desired.
Although no further additives to the polyester layer are necessary, suitable antiblock agents such as zeolites may be advantageously used. Other silicates, clays, talcs, and silicas are suitable antiblock agents, and the antiblock agents are generally used in a concentration of about 500 to about 10,000 ppm (preferably about 500 to about 1500 ppm) based on the weight of polyester.
Other additives, particularly stabilizers, may be used to protect the cap layer from degradation during processing, or to impart other desired attributes to the final film.
The polyester-containing cap layer is added to the core by interdraw coating or lamination. (Interdraw coating or lamination processes are disclosed in U.S. Pat. No. 5,156,904 to Rice et al., the disclosure of which is incorporated herein by reference.) In this method, the core is formed by extruding and casting the polypropylene core, orienting the core in a first (xe2x80x9cmachinexe2x80x9d) direction, forming the polyester-containing outer films, providing the cap layer on one or both sides of the oriented core to produce a monoaxially oriented multilayer film, and orienting the resulting multilayer film in a second direction transverse (and preferably perpendicular) to the first direction.
In practice, a monolayer core or a coextruded laminate of the core polypropylene and a propylene copolymer or terpolymer or linear polyethylene tie layer may be cast onto a roll maintained at a temperature in the range of, e.g., 10xc2x0 C. to 100xc2x0 C., reheated over rolls heated to a temperature (e.g., 100xc2x0 C. to 204xc2x0 C.) high enough to soften the polymer(s) in the core yet below the melting point of the propylene polymer thereof, and then oriented in the machine direction. After the subsequent addition of the cap layer(s), the resulting film is reheated to a temperature preferably higher than the softening point of the outer film layers (e.g., 73xc2x0 C.) and somewhat below the melting point of the core polypropylene (e.g., 150xc2x0 C. to 165xc2x0 C.), and the film is oriented in a second direction transverse (and preferably perpendicular) to the machine direction.
A cap layer may be applied to one or both sides of the core. Similarly, the core may contain a propylene polymer or terpolymer or linear ethylene polymer tie layer on one or both sides of the polypropylene core. The multilayer BOPP structure may but need not be symmetrical; for example, a two-layer cap film may be disposed on one side of the core with a three-layer cap film on the other.
The following non-limiting examples illustrate the practice and benefits of the invention.